With recent development of personal computers, the technique of printing apparatuses has also abruptly advanced. The printing apparatus prints an image on a printing sheet on the basis of image information.
These days, the most attention-getting printing method of the printing apparatus is an inkjet printing method. According to the inkjet printing method, ink is discharged from a printhead onto a printing sheet to print. As the advantages of this method, a high-resolution image can be printed at a high speed, and the inkjet printing method is superior to other printing methods in various aspects such as the running cost and quietness.
There is known a method using an electrothermal transducer which generates thermal energy as ink droplet discharge energy of the inkjet printing method. According to this method, small ink droplets are discharged from small nozzles arranged in an inkjet printhead, thereby printing on a printing medium such as a paper sheet.
Generally in an inkjet printhead using an electrothermal transducer formed from a driving system for forming ink droplets and a supply system for supplying ink to the driving system, the electrothermal transducer is arranged in a pressure chamber, and an electrical pulse serving as printing data is supplied to apply thermal energy to ink. An abrupt phase change in ink at this time, i.e., a bubble pressure generated by gasification is utilized to discharge ink droplets.
The structure of a general inkjet printhead will be described with reference to FIG. 2.
FIG. 2 is a perspective view showing the outer appearance of a general inkjet printhead.
In FIG. 2, the inkjet printhead has nozzle arrays for discharging inks of a plurality of colors. Reference numeral 1 denotes a black (Bk) nozzle array for discharging black ink; 2, a cyan (C) nozzle array for discharging cyan ink; 3, a yellow (Y) nozzle array for discharging yellow ink; and 4, a magenta (M) nozzle array for discharging magenta ink.
The detailed structure of each nozzle array will be explained with reference to FIG. 3.
FIG. 3 is a view showing the layout of the nozzle array of the inkjet printhead.
As shown in FIG. 3, the mainstream of the inkjet printhead is a printhead having a staggered nozzle layout. In FIG. 3, printing main nozzles include 320 black (Bk) nozzles and 128 color (COLOR) nozzles (for each color) (FIG. 3 illustrates nozzles for one of cyan, magenta, and yellow).
The nozzle array of each color is formed from two arrays. In the nozzle array of each color, the left side of FIG. 3 is an EVEN nozzle array, and the right side is an ODD nozzle array.
Each of the EVEN and ODD nozzle arrays has 160 nozzles for black and 64 nozzles for each color.
The positional relationship between nozzles is designed so that two nozzle arrays of the same color each prepared by arraying many nozzles at a predetermined pitch py in the y direction (sub-scanning direction) are spaced apart in the x direction (main scanning direction) by a distance px corresponding to a predetermined number of pixels, and nozzles between the arrays are shifted by (py/2) in the y direction.
The main scanning direction is a direction in which the inkjet printhead is scanned, and the sub-scanning direction is a direction perpendicular to the main scanning direction.
This layout can implement printing at double the resolution of one array by only adjusting the discharge timing between the two nozzle arrays.
The schematic structure of the nozzle array will be explained with reference to FIGS. 4 and 5.
FIG. 4 is a view showing the schematic structure of the nozzle array of the inkjet printhead. FIG. 5 is a sectional view showing the sectional structure of the nozzle array in FIG. 4 taken along the X direction.
Especially, FIG. 4 illustrates the schematic structure of the nozzle array of an inkjet printhead which discharges ink of a predetermined color. In FIG. 4, the nozzle array is formed from a plurality of main nozzles 5 which discharge ink, a plurality of ink chambers 6 in which the main nozzles 5 are open, and an elongated common ink chamber 7 which supplies ink to the ink chambers 6.
An inkjet printhead for a color printer comprises a plurality of nozzle arrays, e.g., four nozzle arrays shown in FIG. 4 in correspondence with four color inks of yellow, magenta, cyan, and black in order to print with multicolor ink. In the above-mentioned inkjet printhead, the main nozzles 5 are arranged at an interval as small as possible in order to downsize the apparatus.
Since the inkjet printhead (to be abbreviated as a printhead hereinafter) treats a liquid, it is applied to an inkjet printer having a suction/recovery mechanism which uses a cap and discharges a liquid within the printhead from the printhead as the liquid thickens, and a preliminary discharge mechanism which drives a driving element (preliminary discharge is executed regardless of a print signal and is also called idle discharge), or having a cleaning mechanism which cleans the nozzle surface.
In order to maintain a good state of the main nozzle 5 of the printhead, the inkjet printer executes “cleaning”, “head refreshing”, and “wiping” as working sequences.
In the first two of these sequences, a negative pressure is applied to the cap which covers the main nozzle 5, and ink in the common ink chamber 7 is sucked to unclog the main nozzle 5. After that, preliminary discharge is performed. By wiping, thickened ink attached to the nozzle surface is removed.
Time-base preliminary discharge is executed at a predetermined time interval even during printing in order to prevent a discharge failure in the next discharge that is caused by thickening of ink in a main nozzle 5 not used for printing upon the lapse of time.
Liquids of different colors or liquids having different characteristics may be mixed between printheads for respective colors regardless of whether nozzle arrays of a plurality of colors are arranged integrally or separately. There are known various means for solving this problem.
Of these means, Japanese Patent Laid-Open No. 8-295033 discloses a technique of preventing color mixing between adjacent printheads by arranging dummy nozzles between them. More specifically, inks from adjacent printheads are guided to the dummy nozzles, and the mixed ink is discharged from the dummy nozzles, thereby removing the mixed ink.
According to Japanese Patent Laid-Open No. 2001-129997, as shown in FIG. 6, some of the main nozzles 5 are used as dummy nozzles 8 along the array direction of the main nozzles 8. Dummy ink chambers 9 of the dummy nozzles 8 communicate with the common ink chamber 7 of the array of the main nozzles 5. In a printhead recovery process, ink staying in the common ink chamber 7 can also be preliminarily discharged from the dummy nozzles 8.
Since bubbles at the two ends of the common ink chamber 7 are discharged from the dummy nozzles 8 together with the liquid, mixed ink can be quickly discharged. In particular, the liquid flow can be promoted between the dummy nozzles 8 and the ends, in the longitudinal direction, of the elongated common ink chamber 7 which receives ink. Thickened ink which tends to stay at the ends of the common ink chamber 7 in the longitudinal direction can be smoothly, reliably discharged outside the printhead from the dummy nozzles 8.
In suction and recovery of the printhead during “cleaning” or “head refreshing” described above, all the nozzle arrays (black, cyan, magenta, and yellow) or the black nozzle array and color nozzle arrays (cyan, magenta, and yellow) are simultaneously sucked through one cap, and all inks are mixed within the cap.
The mixed ink within the cap may be attached to the nozzle surface of the printhead, and after suction operation stops, may be sucked by a negative pressure within the ink tank.
If printing is done in this state, an ink different from an intended one is discharged, greatly degrading the printing quality. In order to prevent this error, mixed ink sucked into the printhead is preliminarily discharged after suction and recovery.
In the common ink chamber 7 shown in FIG. 6, ink tends to stay at the ends in the longitudinal direction because of an elongated structure. To prevent this, preliminary discharge is first performed from the main nozzles 5 used for printing, and then from the dummy nozzles 8 upon the lapse of a predetermined time.
Accordingly, thickened ink at the ends in the longitudinal direction can be discharged from the printhead. As for time-based preliminary discharge during printing, preliminary discharge from the main nozzles 5 used for printing and preliminary discharge from the dummy nozzles 8 are executed.
The state of preliminary discharge by the main nozzles 5 and dummy nozzles 8 will be explained with reference to FIGS. 7A and 7B.
FIGS. 7A and 7B are views schematically showing the state of preliminary discharge.
FIG. 7A shows the state of preliminary discharge from the main nozzles 5, and FIG. 7B shows the state of preliminary discharge from the dummy nozzles 8.
As shown in FIGS. 7A and 7B, when ink is discharged from the main nozzles 5, ink can be satisfactorily discharged from the common ink chamber 7 toward the outlets of the main nozzles, but ink tends to stay at the two ends of the common ink chamber 7. These portions will be called stagnation portions 10.
The stagnation portions 10 readily become sticky owing to stayed ink. To prevent this, ink staying at the stagnation portions 10 is discharged by discharging it from the dummy nozzles 8 after discharge from the main nozzles 5, as shown in FIG. 7B.
As for preliminary discharge, the discharge count for performing discharge is generally equal between the main nozzles 5 and the dummy nozzles 8. Thus, execution of preliminary discharge from the main nozzles 5 used for printing requires preliminary discharge from the dummy nozzles 8 at the same discharge count. A discharge time corresponding to double the preliminary discharge count of the main nozzles is necessary.
As a method of shortening the preliminary discharge time, the discharge frequency of preliminary discharge may be increased. However, the response characteristic of the printhead which performs preliminary discharge is physically limited. The preliminary discharge time cannot be shortened by simply increasing the discharge frequency of preliminary discharge.